GB2388995A

GB2388995A - Piezoelectric sounder excited at dominant resonant frequency whilst driven at lower tonal frequency

Info

Publication number: GB2388995A
Application number: GB0211987A
Authority: GB
Inventors: Michael Barson
Original assignee: Gent Ltd
Current assignee: Novar Systems Ltd
Priority date: 2002-05-23
Filing date: 2002-05-23
Publication date: 2003-11-26
Anticipated expiration: 2022-05-23
Also published as: EP1507603B1; GB0211987D0; EP1507603A1; GB2388995B; AU2003241013A1; ES2527050T3; WO2003099468A1

Abstract

A sounder for an alarm having a piezo-electric element (1) and an electronic drive circuit (100) to drive the piezo-electric element to produce an audible sound perceived as a first tone at a first tonal frequency, the piezo-electric element having a dominant resonant frequency; the dominant resonant frequency is above the first tonal frequency and the electronic drive circuit is arranged to drive the piezoelectric element to produce the first tone sound; the piezo-electric element is also excited at the dominant resonant frequency while the first tone sound is produced.

Description

, 2388995

Improvements in and relating to alarms The present invention relates to alarms having a piez.o-elcctric element for emitting a sourld. - Magnetic buzzers or loudspeakers are used for alarm sounders as these meet the requirement t'or producing a high sound pressure level (Sll.) \vith i'recuency components in the range:()01z to 1000lTz, to comply with the British Standard BS 5g39: Part 1. A problem with these known sounder designs is the high popover 10 consumption, typically > ().5W is required to produce > 10()dE3A. Further, these sounders may also have a high minimum working voltage.

The high current required to power sounders is a major limitation in tire alarm systems. In encral, voltage drops on cables limits the total sounder load and 1- maximum cable nuns possible. This limitation becomes very acute in addressable systems where a large number of both sounders and smoke detectors are required to \\ork on the same cable pair.

2() Piezoelectric elements. maN he used in sounder designs to reduce the power requi rernents.

Most known sounders 'enerate l'reclucncies well Strove 1 KTlz ancl will not tic part of tile tallowing discussion. I\ known tcchnilue <'f'driving a piczoelectric element with a 0: square wave at a third of the resonant frequency is shown in}figure t) and 10. With this known technic, not only is the resonant f'recuency not excited to the same extent (over 6clB down compared to the present invention with the same piezoelectric

( - 2 element), pert'ormance is further degraded as a required warble tone is produced by driving, the piezoelectric element berth sides of the resonant frequency Mote that a higher sound pressure level (SPL) is possible tiffing a nodal mounted piezoelectric element. this also would give the advantage of a feecdlack output from the element (pieoelectric transformer effect) A problem with this known arrangement is that the nodal mount clement will only oscillate at a single high frequency above I Kl l;: and is therefore will not meet the requirements for alarm systems in buildings 1() I'o produce a distinct tonal differences means the Moore away l'rom resonance troth tones must be perceived to be, hence a lower efficienyv and outptt SPIN will be obtained To indicate this. if such a sounder had a warble tone with a tropical 0001 Iz shift in its drive Irequenc, then the higher frequency tonnes would chancre by bOO}Iz, or be off resonance by -()()1 Iz and -3001-lz. which is clearly a prohlell as only a small change in trequeneN will degrade the performance ol'tlle piezo-electrie element and very signifieantlw reduce the sound pressure level producer :O To illustrate this. I igure 9 shows the ringing waveform on the pie;^oeleetric element.

whilst l Inure 1 () sllo\vs the resulting' frequency spectrum produced using a simple square wave drive at a third of the resonant frequency It should Lee noted that the ringing waveform in l:hure 9 is shown in an ideal resonant condition when the piezo electric element is driven by a C) 2211z signal, which cannot occur in practice if 95 different tones arc to be proclcecl 'I'o produce two distinctly separate tones, say 90011% apart, the low tone would be produced hN driving, tire piezo-electric element at X231 Id, and the hi=i1 tone by driving at 102TIz. i''or each of these tcncs the detrimental effect on the amplitude of vibration cuff the piczoelcctric clement, as

! comparecl With the amplitude that would be obtained at the resonant frequency would be very significant, and an inadequate sound pressure level WOUILI be produced.

According to the present invention there is provided an alarm sounder comprising a piezo-electric element and an electronic drive circuit havin'an electrical output to drive the piezo-electric element to produce an audible sound perceived as a first tone, at a first tonal frequency, the piezo-electric element having a clorminant resonant 1() frequency. the dominant resonant Frequency being above the first tonal frequency, the electronic drive circuit being arranged to drive the piezo-elcment to produce the first tone sound, the piezo-electric element being also excited at the dominant resonant Frequency while the first tone sound is produced.

A benefit ol'the piez.o-clectric element being also excited at the dominant resonant 15 frequency is that a high sound pressure level may be obtainecl.

Preferably tile electrical output is further arranged to produce a secoilLI tone, at a second tonal frequency, the first tone frequency beings higher than the second tonal frequency. the piezo-electric element being also excited at the dominant resonant A) t'requency \vhile the second tone is produced.

\ benefit ol' the pie;.o-clectric element being arranged to also produce a sound at a second tonal t'requency while the piezo-electric element ix also being excited at its dominant resonant frequency is that the alarm sounder maw be arranged to produce two alternating suds ol'tlit'f'crent perceived tonal frequencies. having. sinilarly high 2:> sound pressure levels.

I'referahly the electronic drive circuit further comprises a digital signal generator and the electrical output is a digital signal.

( - 4 A benefit of the digital signal generator is that precise control may he obtained over the electrical Output to the piezo-clectric element.

i'ref'erably the digital signal is controlled by a variable scluae ware control frequency, which is a multiple of'the dominant resonant f'requcncy.

\ henct'it of a variable square wave control frequency is that the control electronics may be simplified.

Pret'c rably the electrical Output comprises a digital waveLorn arranged to pulse drive 10 the piezo-electric element in to a constantly reinforced multi-resonant condition A benett -the cligital waveform arranged to pulse drive the piezo-electric element in to a constantly reint'orceci multi-resonant condition or stats is that the sounder is able to produce and maintain the sound at the first or second tonal frequency.

1 > I'reLerahls the electrical output comprises a waveform having, a plurality Ott superimposed frequencies, at Ieast one of the t'requencies having a frequency arranged to stimulate resonance oI the piezo-electric element at the dominant resonant frequency. A benefit of the wavet'orm havin<, a plurality of superimposed frequencies, is that 00 ctit'terent sounds mav the produced. while the at least one hecluency ensures that the dominant resonant t'requencv is stimulatecl thus product a high. s<'und pressure lea el.

I'ref'erably the electronic drive circuit is arranged so that the electrical output 2> comprises a wawef'orm having a plurality ot'superimposcd trequencies, at Ieast one of the frequencies having a frequency arrangecl to stimulate resonance olt}le piez.o

( - 5 electric clement at the dominant resonant frequency, and at least anolllcr of the Frequencies varying with time so as to produce a sounci with a rising or falling tone.

A keenest oi'tlle at least anotller frequency varying, With time while the at least one frequency stimulates resonance ot'thc piezo-electric clement at the dominant resonant 5 Frequency, is that a rising or a f:allin. tone nay he produced \vhile substantially maintaining a high sound pressure level.

A further benefit is that by using complex tlrive waveforms a very low profile fire alarm sounder design may he produced. Such a sounder can produce an output SPL of I O -'I O()dBA. with a rich t'recluencv spectrum, whilst using < 0.1 W of input power.

E'ref'erably the electronic drive circuit is arranged to monitor a dominant resonant response oi'thc piezo-electric clement to the electrical output and is f'urtiler arranged 15 to adjust the square wave control frequency to obtain a maximum dominant resonant response of the Tiez.o-elcctric resonant element.

/\ benefit of the electronic drive circuit beings arranged to n:nitor a dominant resonant response of the piczo-clectric element to the electrical output and being t'urther arranged to adjust the scluare wave control frequency tc' obtain a maximum dominant 0() resonant response of'the piezo-electric resonant element is that any dril't of the actual f'rccluency ot'thc dominant resonant frequency may be cietccted. I {coce, the electronic drive circuit is thus arran'cci to compensate for any change in the actual 1'requcncy at which the ciominant resonance occurs.

95 1ref'crably a sound pressure level produced at the first tone and at the second tone arc within I SdF3 of'each other.

More pret'crably a sound presstre level produced at the first tone anti at the second tone arc within 3dL3 of each other.

( - 6 T'reterably a sound pressure level produced at either tone. is within 1 c113 of a sound pressure level produceci he the picz-clectric element when it is driven try the electrical outptit at a third l-, armonic ot'the dominant resonant l'requency.

More pret-crablv a sound pressure level produced at either tone. is within 1 5dB of a 5 sound pressure level pro-luced bV the piezo-eleetric element when it is driven b!, the electrical Output at a third harmonic ot'the lominant resonant t'requencv.

benefit of the tories having similar high sound pressure levels is that the tones will bc audible,aho\ie an ambient noise IcYcl.

1() Specific embodiments ot'tlle invention mill now be described by wav of example with ret'crence to the aceompanyin drawings in which: Fig,ure I is a schematic circuit diagram for an alarm sounder according to the present I invention.

Figure shows a drive and ringing voltage wavet'orn with respect to lime resulting t'rorn a I I ()()()I 01 (00()1 () I I I).lata sequence generated ciurin, the operation of the circuit ol'Fig,ure 1: ?() 1- inure 3 shows a drive and ringing voltage \vavet'orm Stir respect to time resultinL: I'rom a ()0()1 () 1 data sequence gencrated during the operation of the circuit of leisure 1: li,ure 4 slows a drive vaveforn1 and a resultin,, piezoelectric ringing voltage 25 waveforms with reselect to time generated during the reparation ot' the circuit of Figure

( - 7 Figure 5 shows a DC t'ecUback level, which is monitored by the microcontroller curing initial calibration; igure (I shows an audible hartnonic t'requency spectrum t'or data sequence 1 1000101 1 igure 7 shows an audible harmonic t.'requency spectrum t'or data secluencc 000101 I'iure 8 shows an audible t'retinencv spectrum generated t'rotn the calibration \vavet'orm of'F'igre 4.

l inure 9 silows a kno\vt1 piezoelectric ringing, wavel'ortm obtained if driven by a square wave at a third ot'tle resonant frequency; and I i<'ure 1 () shows a knight l'requency spectrum resultin,, t'rom the square wave shovn i n Fi Inure 9.

rom Figure I an electronic drive circuit 100 for an alarm sounder is shown. the circuit being arranged tc'dtive the piezo-clectric element 10 to produce an audible 9() sound. 'I'hc piezo-electric element has a dominant resonant t'requencv that is stimulated when the audible sound is produced. Whctl resonating at the dominant rcsonatit ttecuency the power consumed by the piezo-electric elcntent for a given sound pressure level is at a minimum. I Ictice, when the electronic drive circuit is driving tile piezo-electric clement 1() to produce a particular sound. the overall sound 5 pressure level may be si,niftcantiv enhanced for a 'iven po\ver consumption' if the piezo-electric element is also excited at the dominant resonant t'requency.

( - 8 A suitable waveform for producing a first audible sound at a high perceived first tone, vhilc also exciting, the piezo-electric element at its cloninant resonant frequency is described with reference to 'inure 3 belov-v. and a suitable wavcf'rTn for protlucin:, a second audible sound at a low perceived second tone while also exciting, the piezo 5 electric element at its Confidant resonant frequency is described with ret'erence to Figure below.

The perceived tone from Figure 3 would include a t'requcncy of 90311z.. while that of Figure 2 would include a frequency of 693 Hz.. Hence, a dift'erence between the two 10 perceived tones would lee 230Hz. and the tones would sound distinctly different.

In a second embodiment oi'thc invention, a measured sound pressure level output from ibe piezo-electric element when driven at resonance by the calibration wavct'orm of Figure 4 was in excess ol' 1 OOdB. A measured output from the piczo-clcctric I element when driven lay the wavcl'orrrl of Figure 3 was less than 3dB lower. When the clement vvas driven by the wavcLorm ot'T inure 2, it was less than Idly lower than the output level produced when driven by the Figure 3 waveform.

In a third cmtodiment ot' the hlvention' the first and second tones produced a sound SO pressure level in excess ol' 1 ()()dl3 t'or an electrical power input ot' less than I OOmW.

For the third cmbodiment a desired electrical power input is less than 75mW.

:hc microcontroller I and the shift rcgistcr'7 and the multiplexer 3 comprise a digital 275 signal Aerator 120. 'I'hc output signal f'rorn the digital signal generator 12() is amplit'ied by the output amplifier 130, which comprises switching transistors X and 9.

( A feedback circuit 140 is provided so that the electronic circuit may be arranged to monitor the dominant frequency response of the piezo- electric element as an output l'reqene,N is varied over a calibration frequency range, peak hold detection circuit I 5() is provicied to enable a peal; resonant response ot'the piezo-clcctric element to be S cletected. AIthough the electronic drive circuit in the embodiment shown and described with reference to Figure 1 uses a digital signal generator to produce a digital waveform arranged to stimulate the piezo-electric element to produce different sounds while also 10 resonating, at the dominant resonant frequency, an alternative embodiment not shown in the f'ig,ures is arranged to provide a suitable \vaveform by superimposing a plurality oi'waveforms front an analO=ue signal generator, so that an output signal is produced that produces an audible sound perceived as a first tone at a first tonal frequency' the piezo-eleetric element having, a dominant resonant frequency, the dominant resonant 15 frequency being above the first tonal frequency, the electronic drive circuit being arranecl to drive the piezo-element to produce the first tone sountl. the piezo-eleetric element being also excited at the dominant resonant frequency while the first tone sound is produced.

:() In the alternative cn1hodinent a suitable feedback circuit may also he provided so that the electrical output iregucncv Irom the analogue drive circuit may be adjusted to ensure it will stimulate the piczo- clectric element to resonate at the dominant resonant l'rctucncy. : An advantage ol'the emUcdiment using a digital signal generator. is that the power supply to the piezo-electric element may be easily produced as a pulsed electrical output, \vhile with an analo'ue signal generator tilt' waveform NN'OUid more easily be produced as a continuous electrical output signal. With a pulSt'd output, a farther hnprovenent in cffciency may be obtained, since the piczo-electric element may be

i - 10 allowed to ring after a pulse, rather than being driven again immediately. and hence electrical power consurllcLl by the piezo-electrie clement and losses in the drive circuit is reclucecl.

5 An advantage of using the shil't register and multiplexer to produce the digital signal.

is that a microprocessor having a low clock frequency may also be used t'or other task SUCil as communication with a remote control panel. i lencc. savings may be made in the power ccnsunption of the sounder, and in overall component costs.

10 'he present invention uses complex drive wavet'onns to pulse drive a piezoelectric element in to a constantly reinforced multi-resonanl condition. 'l'he dominant resonant t'requencN is stimul.ltecl even when the soL,nder produces a \varble tone \vith two clcarl,v distinct tones below I KIlz. A feedback control loop maintains the optimum drive c. 'nclitions ot'the complex waveforms, enablin' a small, e('f'icieat and more 1.. aesthetic sounder design to he produced.

A larvae piezoelectric element (5()mm diameter) that is edge mounted in a ITelmholtz chamber and coupled to a toklcd horn is a practical way ol'producing such a sounder' which has a t'requency response below lKT-lz. .

At) Such a piez.celectric element will have a number o('rcsonant peaks in this arrangement. howeN cr a dominant resonant peak will always csist. 'I'o obtain the lowest possible poNvcr dissipation anal highest possible sound pressure level (SPI.) the pie/.oelectric elenlcnt necd.s to be driven at thi'.. dominant resonant i'reqcncy.'I'his produces a number of' I'undamentcil problems. the first is a requirement that a fire alarm sounder must produce more than a single distinct tone. second problem is that a suitable piezoelectric element with a hi,h SPI. will have its dominant resonant trcqency above I Kl Id. /\ further problem is that the resonant f'requcncy is subject to

( - 1 1 initial manifacturinr tolerances as well as a drift during its useful lifetime due to environmental conditions and ageing I'hese problems arc overcome by aspects of the present invocation.

I'hc generation of the piczoclectric drive waveforms are described Meow in cletail' and shown in Figures:.3 and 4.

When the souncicr is operated to produce the second tone, shown in l:igure 2. 8 bits of 10 ctata are parallel io.deci into an 8-stage shift register A. under the control ol' microcontroller 1. One ol' the outputs Q6 or Q8 of the shift register is selected to feedback into its own serial input (lN). by selecting control line At) of multiplexer 3.

I'he microcontroller I now selects tile serial mode of the shift register 9 using control line 1'/S 'I'hc microcontroller 1, then generates a free runnin,, square wave frequency 15 on port line CLK, whicl1 is used to serially clock the data around the shift register 2, so that it circulates in a continuous loop.

It should be noted that the microcontroller I is able to adjust the square wave l'requency it generates. hence the pulse width of the clocked data bits.

_ () The multiplexor is arran:.ed to circulate a bit data output for the waveform of Fi<,ure 0. and a 6 bit data output tor the waveform ol'Ficure 3.

I'he loop -'t'circulating data forms a complex waveform which we will see contains a 95 tinldamental t'requcocv arid a related harmonic frequency, which is ulthnatelv used to drive the piezoelectric element 10

- 12 Only three waveforms will be examined in any detail although more are clearly Possible. 'I he first wavet'orm shown in Fi=urc 2 is constructed From a data sequence as follows: P8 1'7 1'( T'5 P] P3 P' P 1

1 1 0 () O 1 () 1

In this case the QX output of the shift register 2 is fed back into its own serial input. If we assume the bit pulse width is I8O.SuS. it can be seen that a fundamental frequency I () exist in the waveform. which depends on its cycle period, as t'olkws: 1 / ( 8 bits * 1 8().uS)- 693 FIz 1'here also exists a higher Nth harmonic frequency generated at: 1 / ( bits * 1 80.5uS)= 9770 1-lz I'he second wavef'orn1 to consider shown in Figure 3. is constructed From a data sequence of': Of) P6 I'5 1'4 P3 1' I'1

O O 0 1 0 1

In ll1is case the Qf, <'utput ol'tllc shift register is fled back into its own serial input.

AS Note that in both of' the aboc cases the microcontroller I has set the data bit PS to a logic low.

( - 1 A,

If we again assume that the hit time is 1 80.5us, then a fundamental frequency will now exists at: 1 /( 6 hits * 180.5uS)= 92, 11.

I'here will also exist a higher 3rd harmonic generated at: 1 / (' 2 bits * 1 80.5uS)= 9770 Hz I O The third waveform shown in Figure 4, is constructed from a data sequence as trollops: 1'6 I'5 P4 P3 P2 P 1

1 0 1 1

['his is a simple stluarc wave, which is used t'or initial calibration. In this case the Q6 output is fed back into the shift registers serial input, however tile data bit 1'5 is in this case set to a logic high. Finally if we assume the bit time is attain I SO.5us, then a simple square wave will produce a t'unclamcntal frequency of: 1 / ( ' hits * 1 80.5uSi)-= 2770 1-1.

This is identical to the 3rd and 4th harmonics of the previous complex waveforms, which is in Pact the dominant resonant frequency (fir) Of the piezoelcctric clement.

flee drive wavel'rms Uncrated on output QS, arc applied to switching transistors 8 and 9. Capacitor and resistor 7. form a differentiation network. so that transistor 9 only turns fin during the rising edges of the applied waveforms. Similarly capacitor 4

( - 14 and resistor 6, i'orrn a second network, so that transistor X only turns on during the Calling edges of the applied vvavel'orms. 'the transistors collectors arc connected to the piezoelectric element 10 at point P--, tle other side of the element is connected to ()V ground. Current is now pulsed into and out of the piezoelectric element 10 during the 5 risings and falling edges ol' the drive waveforms.

shifter transistor 8, pulses current into the piezoelectric element 10, the voltage \vill rise to the supply voltage level (Vee) and as transistor 8 turns off the piezoelectrie element I O is then free to resonate. Siirnilarly after transistor 9, pulses current out of I O the piezoelectrie element 10, the voltage across it will fall to zero and the piez,oelectric element 10 is then again Iree to resonate. In this resonant period. a ringing voltage will be produced before transistor 8 turns on attain.

1 i'the applied current pulses occur at the piezoeleetric elements dominant resonant 1:' Frequency (F'r)'thei1 the rinTing voltage on the piezoelectric element 1() after transistor 9 turns on, will ring up to a maximum value. just hcI'ore transistor 8 turns on. 'I'his ringing, voltage, indicated in Figure 4, is sensed by a detection circuit.

peak hold detection circuit is used to produce a OC. volta,e level Shied is 0() monitored by the microcontroller I. on an analo,ue to cligital port (AN l). 'he sounder is initially calibrated during, its manufacturing lay the microcontroller 1 applying waveform A. the simple square wave drive xavci'orm. to the piczoelectric element I O and then frequency sweeping, by adjusting the clock rate ofthe shift register (Cl, K) in distinct frequency steps. The square wave clock duration is increasecl by 2uS at 05 each step. to lower its iretluency and is maintained for 4()mS;. so that the DO level at AN1 is stable enough for analoguc to digital readings to be taken. A wide Irequency capture range is Lised l'or this initial calibration. which is sul'l;cicnt for the expected variance in any piezoelectric elements resonant frequency.

( -]5 The frequency that corresponds to the highest DC level will he the dominant resonant frequency (Fr) ot'the piezoelectric element. The DC level obtaiticd during initial calibration is shown in leisure 5 - Divider resistors 11 and 12 drop down the voltage level applied to the peak detection transistor 14. Resistor 15 and capacitor 16 filter and holct the peak voltage level applied to transistor 14, whilst resistor 17 provides a slow discharc for capacitor 16.

10 As only a part of the voltage can the piezoelectric cicmcnt 10 indicates that it is in a resonant condition. then the rest of the voltage waveform must be blanked off from peak detection transistor 14.

The blanking, network consists of a clamp diode 1- and transistor 21. I'he diode 13 conducts when the drive waef'orms are at a logic low. '(his blanks out the voltage caused by transistor 8 turning, on. 'I''ransistor 21 is also pulsed on to blank the falling edge period of the piezoelectric voltage t'ron1 being applied to transistor 14.

()nce the dominant resonant l'requency (Fr) of the piezoelcctric elertletit 1 () is known 90 then the corresponding; clock rate of the shift register is then stored by the microcontroller 1. Tile microcontroller I from now on will rise this same clock rate? however the complex was et'orms of Figures 2 and ', arc now used to drive the piez'electric element 1() into a multi-resonant condition. The value ol'the DC level is now also stored hv the microcontroller.

If the sounder is switched on, and the resonant frequency has shit'tcd from its initial values the L)C' voltage feedback level to the microcontroller I will have dropped

( - 16 compared to its stored calve. The microcontroller I now executes a mini-resonant search using a complex \vavet'orm to find the optimum operating frequency. If' the DC voltage level has dropped below a Fixed threshold and the sounder is an addressable type, then this Fault condition will be communicated to its control panel.

s From the wavefonns of [:igurcs 2 and 3, we can see that the piezoelectric element 10 is strongly driven at its dominant resonant frequency (Er). which is the 4th harmonic and 3rd harmonic respectively of the complex drive vavet'orms basic t'recuencies.

I () The microcontroller I is able to switch between the two complex drive waveforms to produce a warble tone. Fixtures 6 and 7 show the t'requency spectrum produced in each case. What is clear, is that a very rich harmonic frequency spectrum is produced in both cases, whilst the same dominant resonant frequency (Fr) has been generated. 'I'his gives maximum eff'iciencv with two videly separated low trequencv tones in the 15 range OOHz to I KHz Figure shoves the frequency spectrum produced due to the calibration drive waveform of'Figure 4. Tote that the peak output is always at the same dominant resonant frequency (Fr) in all cases.

It will be appreciated that. although described with reference to sounders. and particularly to sounders tor alarm systems in buillings the invention is also suitable for use with alann sounders for use with vehicle alarms' or souilLiers.sLich as are used tor warning devices t'or vehicles' For instance as rever.sing \\aning sounders. or 2: emergency service vehicle sounders.

Claims

( - 17 C,.LAIMS

1. A sounder comprising a piezo-electric element and an electronic drive circuit having an electrical output to drive the piezo-electric element to produce an audible sound perceived as a first tone. at a first tonal frequency, the piezo-clectric element having a dominant resonant frequency, the dominant resonant frequency being above the First tonal frequency, the electronic drive circuit being arranged to drive the piezo-

element to produce the first tone sound, the piezo-electric element being also excited at the dominant resonant frequency while the first tone sound is produced.

It) 9. A sounder as claimed in claim I, wherein the electrical output is further arranged to produce a second tone. at a second tonal t'rccJuency. the first tone freqtency being higher than the second tonal frequency. the Tiezo-electric element being also excited at the ctominant resonant t'requencv while the second tone is produced. 3. A sounder as claimed in clean I or 2, wherein the electronic drive circuit further comprises a digital signal Decorator and the electrical output is a digital signal.

4. A sounder as claimed in claim it.. wherein the cligital signal is controllect by a 0 variable square wave control frequency, which is a multiple of the dominant resonant frequency. >. sounder as claimed in claim 4, wherein the elf ctronic drive circuit is arrangecl to monitor a dominant resonant response ot'the piezc>-electric element to the electrical output and is t'urther arranged to adjust the sqtrare wave control frequency to obtain a maximum dominant resonant response ot' the piczo-electric resonant element.

- 18 6. A sounder as claimed in any of the preceding claims. wherein the dominant resonant l'requency is above I Katz.

7. A sounder as claimed in an; of the preceding claims. wherein the tonal 5 f'rec:luency is within a 1'reqicncy range between:001 Iz. and I kl Iz.

8. A sounder as claimed in claimed in any of claims 5 to 7 when dependent on claim 5, wherein the electronic drive circuit is arranged to adjust the square wave control frequency within a narrow range while the first or second tone is being 10 sounded to discover the frequency of the dominant resonant response.

9. sounder as claimed in claim 8. wherein the electronic drive circuit is further arrangecl to monitor a l'ecdhacli voltage level from the piezoclectric element while the piczo-clectric element is heing sounded, and to compare the t'cedhacl; voltage level 15 with an acceptable minimum voltage level.

I O A sounder, substantially as hereinbefore described and with reference to the accompanying drawings numbered from Figure I to Figure 8.